Polytetrafluoroethylene coating compositions



Patented June 15, 1954 POLYTETRAFLUOROE THYLEN E COATING COMBO SITION S Seymore Hochberg, Upper Darby, Pa., assignor to E. I.

du Pont de Nemours and Company,

Wilmington, Del., a corporation of Delaware No Drawing. Application August 9, 1951,

Serial No. 241,170

This invention relates to liquid polytetrafluoroethylene coating compositions and more particularly to liquid polytetrafiuoroethylene coating compositions which yield relatively thick crackfree films.

A customary and convenient form of liquid polytetrafiuoroethylene coating composition is a dispersion of colloidal polytetrafiuoroethylene in water as disclosed in U. S. Patent 2,478,229 to Berry and U. S. Patent 2,534,058 to Renfrew.

It has long been known that polytetrafiuoroethylene films which are deposited on a substrate by a single application of an aqueous suspension of polytetrafiuoroethylene tend to develop cracks during the drying and baking steps when the dry film thickness exceeds the order of 1 to 2.5 mils depending upon the particular lot of aqueous polytetrafluoroethylene suspensoid used. Each different lot of suspensoid possesses its own critical cracking thickness value which is the dry film thickness above which objectionable cracks develop during drying and/or fusing. The cracks formed during drying of the polytetrafiuoroethylene aqueous suspensoid vary in size from unmagnified visual cracks to microscopic cracks.

This cracking tends to develop not only in films of uniform thickness greater than about 1 to 2.5 mils but also where such a film thickness occurs accidentally because of poor control in application or because of the shape of the object being coated. For instance, a smooth steel panel coated with a particular sample of an aqueous polytetrafiuoroethylene suspension at a dry film thickness of 1.5 mils over most of the panel area might be free of cracks except for narrow areas along the periphery where a fatty edge or bead had formed. Such a bead might be 2-3 mils thick, resulting from a normal accumulation of excess coating material along the edges of the article being coated by conventional means. Cracks in a polytetrafiuoroethylene coating could also be expected on the convex or high areas of an irregularly shaped article. This phenomenon is illustrated in applying a coating of an aqueous suspension of polytetrafiuoroethylene to the mold surfaces of a muffin tin. In attempting to apply a uniform 1 mil coating on the indented mold surfaces, a considerable thickness of polytetrafiuoroethylene is accidentally built up on the upper edge or rim of each mufiin mold. When 12 Claims. (Cl. 260-2915) ,fiuoroethylene coating compositions heated, to dry and fuse, the coating develops cracks at this point. Such cracks not only adversely affect the appearance of the article but they lead to premature failure of the coating.

For certain purposes, particularly in corrosion resistant service on metals, it is necessary or desirable to have polytetrafiuoroethylene films more than 2.5 mils thick and sometimes even as high as 5 to 15 mils thick. Under very closely controlled conditions, relatively thick polytetrafluoroethylene films can be built up on a substrate by applying several consecutive thin coats of an aqueous suspension, i. e. of the order of 1 mil each, followed by a separate baking or fusing operation after each coat. It is obvious however that this method is laborious, cumbersome and costly; and furthermore it does not eliminate the need for extreme care in avoiding heavy beads or fatty edges.

U. S. Patent 2,539,329 which issued January 23, 1951, to P. F. Sanders discloses a mechanical method of eliminating cracks in polytetrafluoroethylene coatings applied from an aqueous suspension to inorganic fabrics, such as, e. g. woven glass fabrics, but it is obvious that such a process is not practical for irregularly shaped rigid articles, such as the above mentioned muffin tin.

The principal object of this invention is to increase the critical cracking thickness of a film deposited from a single coat of an aqueous polytetrafiuoroethylene coating composition.

Another object of this invention is to provide modified aqueous polytetrafluoroethylene coating compositions which yield in one coat crackfree films substantially thicker than those obtainable with polytetrafiuoroethylene alone in one coat.

A further object is to provide liquid polytetrawhich do not crack when applied at dry film thicknesses above the order of 1 to 2.5 mils, this thickness being representative of dry film thickness at which films of polytetrafluoroethylene alone can be expected to crack.

A still further object is to provide a liquid polytetrafiuoroethylene coating composition which can be applied to a substrate in one coat to yield a dry crack-free protective coating which is substantially thicker than the thickest dry crackfree film capable of being produced by a par free of cracks ticular aqueous polytetrailuoroethylene suspension, the critical cracking thickness of which is to be improved.

Another object is to provide articles coated with thick crack-free polytetrailuoroethylene coatings.

These objects are accomplished by incorporating into an aqueous dispersion of colloidal polytetrafiuoroethylene a substantial amount of a modifying polymeric material other than polytetrar'luoroethylene in the form of discrete particles suspended in a liquid medium comprising a preponderant amount of water, and/or solventswollen particles dispersed in a liquid medium comprising a substantial proportion of water and/or globules of an organic solution dispersed in a liquid medium comprising a substantial proportion of water. The modifying polymeric material in said form must be co-dispersible and compatible with the polytetrafiuoroethylene aqueous suspension and stable for a sufficiently long period of time when admixed with the polytetrafiuoroethylene suspension to be commercially useful. The modifying polymeric material in said form must also be capable alone of yielding in one coat a crank-free film thicker than the thickest one coat film capable of being produced by the aqueous polytetrafiuoroethylene suspension alone, when said modifying polymeric material is baked at the fusion or coalescing temperature of the admixture of the polytetrafiuoroethylene and the modifying polymeric material.

The mixed dispersion of polytetrafiuoroethylene and other polymeric material is referred to hereinafter as a codispersion. The modifying polymeric material which is admixed with the polytetrafluoroethylene dispersion is referred to hereinafter as polymeric material or codispersed polymeric material or codispersed material. The term critical cracking thickness as used throughout the specification and claims means the dry film thickness above which cracks develop in a film deposited from one coat of an aqueous dispersion of polytetrafluoroethylene. Throughout the specification and claims the terms dispersion and suspension are used synonymously to denote a composition which contains discrete particles comprising polymeric material distributed uniformly throughout a liquid medium. The term crack means a linear rupture or fissure which extends from the outer surface of a film partially or completely through the film to the substrate beneath, such fissure being at least large enough to be visible to the naked eye or when magnified one hundred times under good illumination. The terms "crack-free and mean that the film in question of not more than one of the for each 100 sq. centimeters contains an average above defined cracks of film surface area.

The method of determining whether a polymeric material is capable of forming a crackiree film in one coat as defined above, having the thickness required of the ultimate polytetra- 'fluoroethylene coating, consists of (1) applying to a smooth rigid substrate, such as a clean 4 by 4: glass panel, by spraying, dipping or other suitable method, enough of a liquid coating composition containing the polymer in question to yield a dry film of the desired thickness, (2) drying the film at about 100 C. until substantially all of the volatile component has evaporated, (3) baking or fusing at the required temperature and l) examining the coating by the naked eye, or under magnification if necessary, to determine 4 the presence or absence of cracks as defined above. 7

The following detailed description is given by way of illustration and not of limitation. The parts and percentages are on a weight basis.

EXAMPLE. 1

A relatively concentrated aqueous suspensoid of polytetrafiuoroethylene having the following composition was used: 7 I,

. Per cent Colloidal polytetrafiuoroethylene 60.0

Wetting agent 0.6

Water 39.4

This aqueous suspensoid had been prepared by the methods disclosed in Renfrew U. S. Patent 2,534,058 which issued December 12, 1950, and Barry U. S. Patent 2,478,229 which issued August 9, 1949.

The wetting agent was the sodium salt of the sulfuric acid ester of a mixture of long chain alcohols, predominantly lauryl alcohol. Other suitable wetting agents may be substituted in quantities sufficient to stabilize the dispersion. Aqueous suspensoids containing different concentrations of polytetrafluoroethylene may be substituted for the 60% suspensoid on the basis of equivalent polytetrafluoroethylene content. Concentrations between about 5% and are useful. The 60% material was used largely for its convenience of handling and its high solids content which is conducive to thicker dry films than lower solids dispersions.

To parts of this aqueous polytetrafiuoroethylene suspensoid were slowly added 21 parts of Rhoplex lVlR aqueous suspension of polymeric lower alkyl acrylate (47.6% solids) with moderate agitation. Agitation was continued for about 5 minutes to assure uniformity. One face of clean 4 by 4 steel panel was sprayed with a heavy coat of the resulting codispersion. The panel was force dried for 10 minutes at about 212 F. and then baked for 5 minutes at 50 F. The force drying step was employed only to drive off the water before rusting of the steel could take place. If an inert substrate, such as glass, had been used, the water in the film could have been substantially removed by a longer drying period at room temperature. This pre-drying step provides a factor of safety in eliminating the possibility of rough or blistered films which might result from the rapid volatilization of large quantities of water at high baking temperatures.

The dry coating of codispersion on the steel panel was measured and found to be (s5 mils thick. It was smooth and crack-free. Comparative coatings derived from the aqueous polytetrafiuoroethylene suspensoid alone were smooth and crack-free at 1 mil dry film thickness, sparsely populated with small cracks at 2 mils, and more densely populated with relatively large cracks at 3 mils. The critical cracking thickness for this particular lot of polytetrafiuoroethylene aqueous suspensoid was 1.8 mils. i

These results show that films derived from an aqueous codispersion of Rhoplex MR and polytetrafiuoroethylene are crack-free at greater thicknesses than the polytetrafiuoroethylene alone. In investigating the reason for this improvement, Rhoplex MR films were prepared by the. methods previously described. It was (1- 6 termined that the Rhoplex MR alone formed to be 21 to 150; Example 2, 69 to 109; Example 3,

crack-free films up to 4.5 mils thick. 52 to 125; Example 4, 13 to 157, etc. In Examples Rhoplex MR was tested at difierent concen- 17, 20 and 27, the organic solvent may be added trations and other aqueous polymeric dispersions to either or both aqueous dispersions before were blended with aqueous polytetrafiuoroethyl 5 they are combined or to the combined dispersions. ene suspension and tested as described in the It is obvious from the results recorded above above example. l he results are recorded as E-xthat this invention is not defined by the chemi amples 1 through 31 in the following Table I. cal constituents of the codispersed polymeric ma- For convenience in making direct comparisons, terial. For instance, Examples 13, 14 and 15 are the same lot of polytetrafiuoroethylene disperw all based. on styrene/butadiene copolymer latices sion was used in all examples. Since it had a but Example 14 is inoperable within the spirit critical cracking thickness of 1.8 mils, an arbiand scope of this invention. Examples 18 and 19 trary value of 2 mils (approximately 10% imare both based on butadiene/acrylonitrile copolyprovement) was established as the minimum film mer latices but Example 19 is inoperable. Exthickness for acceptable codispersed polymeric 15 amples 23 and 24 are both based on polyvinyl material and codispersions thereof with this paracetate latices but Example 23 is inoperable. Exticular lot of polytetrafluoroethylene amples 28 and 29 are both based on polyamide Table I CRACKING PROPERTIES OF FILMS Aqueous Poly- Aqueous Polymeric Dispersion gg gig Codispersion Dispersion m F Page] Added amp e orms o ven orms Parts Parts Crack- Crack-- Name Polymeric Constituents Polyg ggi tetrag Thick- Medi um Mils or fiuom' M'l 2 Hess Thicker ethylene f r Mils cker m v Rhoplex MR Alkyl Acrylate. 10 11 Yes.-. 90 4. ...do do 33 36 Yes... 67 3 do-. 25 27 Yes. 75 5. 4 4 5 8 Yes. 95 5. 5 5 0 28 42 72 9. 5 6 Rhoplex W09 5 8 95 4 2 o 28 43 72 13, 7 8 Rhoplex WN 75.. 28 42 72 5 9. "Rlioplex E R. 33 34 67 5 10 Rlioplet WN80.. 38 42 72 5 11 Polybutyl acrylate. ate. 22 34 78 10 12 "Methacrol LB... Butyl Methacrylate. 22 46 78 10 13 Dow Latex 512K Styrene/butadienc. 32 39 68 10 14 Fllolite 5 Latex 190... do 23 55 77 2 15 Lotol 5229 do. 80 7 5 16 Bakelite BKSQZ Styrene. 18 26 82 2 17 o ....d0 18 26 82 5 Nitrex" 2612...- Butadiene/acrylo 22 78 13 Hycar Latex 2507... do 16 26 84 2 ....do do.. 16 26 84 2 Polyaorylonitrile Acrylonitrlle 28 42 72 2 Polymethacrylonitrile.. Methacrylomtr 19 67 S1 2 Elvacet" 10 81-900..... Vinyl acetate 33 27 67 2 "Elvacet 83-1200 do 23 19 77 2 Geon Latex 31X.... Modified Vinyl 0 oride. 22 57 78 2 Dow Latex 744 12 Vinyl chlorlde/vlnyl- 22 22 78 2 idene chloride. 27 ..do .110 22 22 i4 methyl iso- Yes 3 5 butylketone. 28 Polyamide A-000 Amine-acid condensate. 22 22 29 Polyamide B-200 14 d0 22 22 30 Polythene Sulfonamide. Sultihoniimide of poly- 29 e y one. 31 Polythene Ethylene 35 1 Bakelite BKSQZ is used by the Bakelite Division, Union Carbide & Carbon Corporation to designate an aqueous dispersion of what is understood to be polystyrene.

5 Nitrex is a trade name used by the Naugatuck Chemical Division, United States Rubber Company for aqueous dispersions of what are understood to be copolymers of butadiene and acrylonitrile.

P Hycar is a trade name used by B. F. Cioodrich Chemical Co. for rubbery oopolymers of butadiene and acrylonitrile.

12 Dow Latex 74 4 is used by Dow Chemical Company to designate an aqueous dispersion of what is understood to be a copolymer of vinyl chloride and vinylide ne chloride.

l3 Polamide A000 1s a trade name used by General Mills, Inc., for aqueous dispersions prepared by the condensation of dimerized vegetable oil acids and ethylene diamine. The suspension is fortified with small amounts of modifying resin.

14 Ifolyamide 13-200 is a tradename used by General Mills, Inc., for an aqueous dispersion of a condensation polymer derived from ethylene diamine and dimerlc vegetable oil acids which has been further compounded aiter dispersion to improve adhesive characteristics.

In the above table the ratio of the aqueous latices, but Example 28 is inoperable. polymeric dispersion to the aqueous polytetra- Furthermore the results recorded in Table I fluoroethylene dispersion is shown for Example 1 show that, if a polymeric dispersion does not itself form a crack-free film as defined, the codispersion with polytetrafluoroethylene does not yield such a film. Examples 14, 16, 19, 21, 22, 23, 26, 23, and 31 show this result. However Examples 17, 20 and 2'7 show that, if a polymeric dispersion which does not itself form a crackfree film above the critical cracking thickness of the polytetrafluoroethylene, as defined, is modilied to make it form such a film, a similarly modified codispersion of the samepolymeric material with polytetrafluoroethylene does form a crackfree film.

EXAMPLE 32 An aqueous suspensoid of polytetrafiuoroethylene having the following composition was used:

Parts Colloidal polytetrafluoroethylene 50.0 Wetting agent Water 49.5

This suspensoid differed from the polytetrafiuoroethylene suspensoid used in Example 1 only in the concentrations of polytetrafiuoroethylene and wetting agent.

To 100 parts of the above suspensoid were added 10.5 parts of Table I) with moderate agitation. Mixing was continued until the codispersion was uniform.

Two swatches of woven glass fabric were out from a roll or" Owens-Corning Fiberglas #ECC-108, as described on page 28 of the Owens- Corning Fiberglas Corporation catalog entitled Manual and Catalog of Fiberglas. The EICC-108 Fiberglas had a thickness of 2 mils.

One swatch was coated with the above codispersion by applying two separate coats by dipping, each coat being followed by force drying at 3l0-330 F. After the second force drying step the assembly was heated at 750 F. for about 5 minutes to fuse or sinter the coating. The resulting coated fabric had a thickness of about 4.5 mils and showed no cracks.

The other swatch was coated by the same process with two coats of the polytetrafiuoroethylene aqueous suspensoid, without Rhoplex ER. The resulting coated fabric was characterized by the presence of mud-cracks as described in Sanders U. S. Patent 2,539,329 which issued January 23, 1951.

These results show that crack-free modified polytetrafiuoroethylene coated fabrics can be made by a simple coating and fusing process which eliminates the calendering step described in Sanders U. S. Patent 2,539,329.

Additional tests on the coatings derived from the operable examples of the codispersions show that the crack-free coatings also possess the desirable properties of coatings derived from polytetrafiuoroethylene alone, namely, good electrical properties, chemical and solvent resistance, corrosion resistance, and resistance to the adhesion or sticking of extrinsic substances applied thereto.

In order to define the invention, attempts were made to distinguish the operable polymers from the inoperable ones by such properties as their chemical composition, molecular weight, melting or fusing point, viscosity, solubility, particle size, and physical form or state. A cataloging of such properties provided no line of distinction by which the operable polymers could be distin- Rhoplex ER (footnote 2,

guished from the inoperable ones. The only method for accomplishing this is to employ the physical procedure of determining whether the polymeric dispersion in question by itself forms a crack-free film of the necessary thickness as V defined above.

. mer to polytetrafiuoroethylene ranges between 5 95 and 33/67 It is to be emphasized that these ratios are merely illustrative. Since the many operable codispersed materials possess desirable properties to different degrees, it is obvious that when all are separately codispersed with polytetrafiuoroethylene in exactly the same ratio, the resulting codispersions do not have the same properties. For example, although 5% of one codispersed polymeric material may be sufficient to produce acceptable results, 10% of a different codispersed polymeric material may be required to provide equivalent results at the same film thickness under the same conditions. Since the coating compositions of the invention are designed for a variety of uses which require different film thicknesses, the minimum amount of any operable codispersed material to be compounded with polytctrafluoroethylene is that amount which is sufficient to produce a crackfree coating having a film thickness greater than the critical cracking thickness of the polytetrafluoroethylene to be modified. The optimum amount of any operable ccdispersed material to be compounded with polytetrafluoroethylene is that amount beyond which no additional advantage is obtained at the film thickness required for the use to which it is to be put.

In the examples a single baking schedule was used, namely, 5 minutes at 750 F., largely as a matter of convenience to eliminate the baking variable which might result in differences in performance of the coatings. The particular length of time and the particular temperature form no part of this invention and are not critical. The

time and temperature, in combination, may be varied over wide ranges from the minimum combination required to produce the desired film up to the maximum beyond which decomposition, charring, or other deleterious effects are produced.

In Examples 1-31, the test anels are coated by spraying. Other well-known methods of application may be employed such as, for instance,

dipping, fiowing (pouring and draining) and electrodeposition.

In the examples the codispersions consist of an aqueous dispersion of polytetrafluoroethylene and one other aqueous dispersion of polymeric material. The-invention is not limited thereto, as it is obvious that polytetrafluoroethylene dispersion may be modified with more than one other dispersion and/or a dispersion containing two or more distinct polymeric or copolymeric materials, provided of course that the codispersed materials have the properties.

agents, such as pigments,

previously defined physical The polytetrafiuoroethylene dispersion and/or the codispersed material may contain modifying dyes, soluble chemical substances, inhibitors, dispersing agents, and other modifiers well-known in the coating composition art, to color, stabilize or otherwise modify the chemical or physical properties of the codispersions or the films derived therefrom, provided any such modifying agent employed is innocuous to the composition and its components.

The products of this invention are particularly useful in coating heat resistant surfaces, such as, metals, ceramics, glass fabrics, asbestos fabrics, woven wire fabrics, heat treated polyacrylonitrile fabrics such as disclosed in copending application S. N. 706,515, filed October 29, 1946, by C. R. Humphreys. The coating compositions of this invention are also useful for coating substrates which have a decomposition temperature below the fusion temperature of polytetrafiuoroethylene, such as, cotton, nylon, rayon, and various synthetic resins, in which case the baking temperature must be below the fusion temperature of the substrate.

Specific uses for the products of this invention include anti-sticking coatings for muffin tins or cookie sheets, electrically insulating coatings for wire, spark-plugs, condensers and corrosion resistant interior coatings for metal tanks containing corrosive chemical materials.

Although the invention permits polytetrafluoroethylene to be used in film thicknesses commonly employed in the paint, varnish, enamel, coated fabric, and unsupported sheeting industries, it is not to be understood as providing articles or coatings so thick that they are not normally useful in one of the aforementioned industries.

It is apparent that many widely different embodiments of this invention can be made without departing from the spirit and scope thereof and, therefore, it is not intended to be limited except as defined in the appended claims.

I claim:

1. A liquid coating composition which comprises an aqueous dispersion of polytetrafluoroethylene containing a wetting agent and an aqueous dispersion of another polymeric material selected from the class consisting of polymeric alkyl acrylates in which the alkyl group contains less than carbon atoms, polybutyl meth acrylate, copolymers of styrene and butadiene, r

polystyrene, copolymers of butadiene and acrylonitrile, and copolymers of vinyl chloride and vinylidene chloride, the said second mentioned aqueous dispersion of polymeric material and said coating composition each having the property of drying at a temperature of at least 100 C. in one coat to a crack-free film having a thickness greater than the critical cracking thickness of said aqueous dispersion of polytetrafiuoroethylene alone.

2. A liquid coating composition comprising an aqueous dispersion of polytetrafluoroethylene containing a wetting agent and an aqueous dispersion of a polymeric alkyl acrylate in which the alkyl group contains less than 5 carbon atoms, the second mentioned aqueous dispersion and said coating composition each having the property of drying at a temperature of at least 160 C. in one coat to a crack-free film having a thickness greater than the critical cracking 10 thickness of said aqueous dispersion of polytetrafluoroethylene alone.

3. A liquid coating composition comprising an aqueous dispersion of polytetrafiuoroethylene containing a wetting agent and an aqueous dispersion of a copolymer of butadiene and acrylonitrile, the second mentioned aqueous dispersion and said coating composition each having the property of drying at a temperature of at least C. in one coat to a crack-free film having a thickness greater than the critical cracking thickness of said aqueous dispersion of polytetrafiuoroethylene alone.

4. A liquid coating composition comprising an aqueous dispersion of polytetrafluoroethylene containing a wetting agent and an aqueous dispersion of a p-olybutyl methacrylate, the second mentioned aqueous dispersion and said coating composition having the property of drying each at a temperature of at least 100 C. in one coat to a crack-free film having a thickness greater than the critical cracking thickness of said aqueous dispersion of polytetrafiuoroethylene alone.

5. A liquid coating composition comprising an aqueous dispersion of polytetrafiuoroethylene containin a wetting agent and an aqueous dispersion of a polyvinyl acetate, the second mentioned aqueous dispersion and said coating composition each having the property of drying at a temperature of at least 100 C. in one coat to a crack-free film having a thickness greater than the critical cracking thickness of said aqueous dispersion of polytetrafiuoroethylene alone.

6. A liquid coating composition comprising an aqueous dispersion of polytetrafluoroethylene containing a wetting agent and an aqueous dispersion of a copolymer of butadiene and styrene, the second mentioned aqueous dispersion and said coating composition each having the property of drying at a temperature of at least 100 C. in one coat to a crack-free film having a thickness greater than the critical cracking thickness of said aqueous dispersion of said polytetra-fiuoroethylene alone.

7. An article coated with a dry crack-free coating of the composition of claim 10.

8. A rigid article coated with a dry crack-free coating of th composition of claim 10.

9. A flexible substrate coated with a dry crackfree coating of the composition of claim 10.

10. A liquid coating composition comprising an aqueous dispersion of polytetrafiuoroethylene containing a wetting agent and an aqueous dispersion of another resinous polymerized ethylenically unsaturated material, the second mentioned aqueous dispersion and said coating composition each having the property of drying at a temperature of at least 100 C. in one coat to a crack-free film having a thickness greater than the critical cracking thickness of said aqueous dispersion of polytetrafiuoroethylene alone.

11. The method of increasing the critical cracking thickness of an aqueous polytetrafluorc ethylene coating composition which comprises incorporating in an aqueous dispersion of polytetrafiuoroethylene containing a wetting agent a compatible aqueous dispersion of another resinous polymerized ethylenically unsaturated material, the second mentioned aqueous dispersion and said coating composition each having the property of drying at a temperature of at least 100 C. in one coat to a crack-free film having a thickness greater than the critical cracking thickness of said aqueous dispersion of polytetrafluoroethylene alone.

11 1.2. The method of producing crack-free modified polytetrafiuoroethylene films in one coat which are substantially thicker than the thickest dry crack-free film which can be produced by an aqueous dispersion of polytetrafluoroethylene alone in one coat, which comprises blending an aqueous dispersion of polytetrafluoroetnylene containing a wetting agent with a compatible aqueous dispersion of another resinous polymerized ethylenically unsaturated material, ap-

plying the resulting blend to a substrate in the form of a film, and drying, the second mentioned aqueous dispersion and said blend each having the property of drying at a temperature of at least 100 Cjin one coat to a crack-free film having a thickness greater than they critical cracking thickness of said aqueous dispersion of polytetrafiuoroethylene alone.

No references cited. 

10. A LIQUID COATING COMPOSITION COMPRISING AN AQUEOUS DISPERSION OF POLYTETRAFLUOROETHYLENE CONTAINING A WETTING AGENT AND AN AQUEOUS DISPERSION OF ANOTHER RESINOUS POLYMERIZED ETHYLENICALLY UNSATURATED MATERIAL, THE SECOND MENTIONED AQUEOUS DISPERSION AND SAID COATING COMPOSITION EACH HAVING THE PROPERTY OF DRYING AT A TEMPERATURE OF AT LEAST 100* C. IN ONE COAT TO A CRACK-FREE FILM HAVING A THICKNESS GREATER THAN THE CRITICAL CRACKING THICKNESS OF SAID AQUEOUS DISPERSION OF POLYTETRAFLUOROETHYLENE ALONE. 